1. Field of the Invention
The invention relates generally to methods of detecting contaminants, and more particularly to a method of detecting contaminants in a food sample using THz spectroscopy.
2. Description of the Related Art
Recent recalls involving pet food and milk products contaminated with melamine and other contaminants have created a widespread food safety scare. This type of contamination of food products can lead to disease or death in people and animals. Recently, for example, melamine had been found in infant formula, powdered milk, pet and animal food, and other protein-based food commodities. The melamine in infant formula, for example, resulted in deaths in China. The result has been increased awareness of the hazards of melamine and a heightened need to detect its presence and other food contaminants.
Compositionally, melamine (2,4,6-triamino-1,3,5-triazine) is an organic base and a trimer of cyanamide. It has several industrial uses, including fire retardation, fertilizer, and in the production of plastics, glues, and laminates. More recently, however, melamine has been used illegally to increase the apparent protein content of food products. For example, for protein analysis tests such as Kjeldahl or Dumas, which are based on nitrogen content, some have illegally added melamine to food because melamine is rich in nitrogen.
Of course, melamine consumption can be toxic in high doses. It is thought that simultaneous ingestion of melamine and one of its analogues, cyanuric acid, may interact in the urine-filled renal microtubes and may result the formation of round, yellow crystals. These crystals block and damage the renal cells that line the microtubes, causing the kidneys to malfunction.
Given the hazards associated with melamine contamination of food products, the Food and Drug Administration (FDA) has currently imposed a limit of 1 ppm of melamine in infant formula and 2.5 ppm in other foods. The European Union (EU) has set a standard of 0.5 milligrams of melamine per kilogram body mass. In light of such stringent regulation, detection of melamine at very low concentrations is critical.
The FDA lists two principle methods for detecting melamine in food products: liquid chromatography triple quadrupole tandem mass spectroscopy (LC-MS/MS); and gas chromatography/mass spectroscopy (GC-MS). See U.S. F.D.A., Library Information Bulletin Nos. 4421 & 4423, Volume 24, October 2008. The LC-MS/MS method detects the presence of cyanuric acid and melamine in infant formula. The GC-MS method detects the presence of melamine, ammeline, ammelide, and cyanuric acid in dry protein materials. Both methods involve many steps, including extraction, filtration, centrifugation, and dilution or evaporation, after which the treated sample is then analyzed using a liquid or gas chromatography column. The LC-MS/MS method is able to screen for contaminants at concentrations as low as 0.25 μg of melamine per gram of dry infant formula. The GC-MS method can screen for the melamine and its analogs at concentrations as low as 2.5 μg of contaminant per gram of dry protein material.
While effective in identifying melamine at low concentration levels, these methods are not necessarily ideal for high-throughput testing situations, due to the process time and requirement for numerous sample treatment steps. Also, sample preparation is time-consuming and labor-intensive, as are the cleanup procedures.
Recently, near- and mid-infrared spectroscopy techniques have been used for melamine detection, in particular in infant formula powder. See Mauer et al., J. Agric. Food Chem., 57:3974-3980 (2009). The techniques are purportedly able to reach the current FDA limit of 1 ppm, but they do not appear to be as sensitive as LC-MS/MS and GC-MS, which can achieve parts-per-billion (ppb) sensitivities. While not as sensitive, these techniques offer some advantages because they are able to detect melamine much faster than LC-MS/MS and GC-MS, and with generally fewer processing steps. Overall though, near- and mid-infrared spectroscopy are still not a complete solutions to the need for high sensitivity, high throughput, rapid melamine detection. For example, there are a number of materials that can block near infrared radiation, and thus hinder detection of contaminated products hidden underneath certain materials. Furthermore, at near- and mid-infrared wavelengths, there can be scattering variations in spectrometer response that are dependent upon the particle size and shape of the powder under analysis. To counteract this, the proposed near- and mid-infrared techniques use a scatter correction process, along with a partial least-squares model to analyze the spectroscopy data. A similarly rapid, but more sensitive, and less limited technique for food contaminant detection is still desired.